Cooking a food in a sealed plastic bag is referred to as “Sous Vide”, and was invented in 1974 by the french. It can also be performed in a glass jar, so we definitely could remove the plastic from the equation, but there are “food safe plastics” which have been demonstrated to have no known health issues when used for this purpose.

Some plastics, like BPA or PVC, are dangerous to consume/do easily leach into food/water, but “plastic” is a very broad term that refers to a lot of different materials.

Note: microplastics are a whole different story, and we’re not really sure how bad they are for you. It is perfectly reasonable to ask the question, but society at large has essentially decided the convenience outweighs the risk, and good luck trying to avoid it in your food.